It has heretofore been proposed to provide a plastic closure for a container that comprises a plastic cap or shell with an interior liner for sealing engagement with the sealing surface of the container. For example, U.S. Pat. No. 4,984,703 discloses a plastic closure that comprises a shell having a base wall with a peripheral skirt and a thread for securing the closure to a container, and a sealing liner compression molded in situ on the interior of the shell base wall. The sealing liner comprises a blend of ethylene vinyl acetate (EVA) and a thermoplastic elastomer material such as olefin or styrene-butadiene-styrene (SBS). U.S. Pat. 5,451,360 discloses a method and apparatus for compression molding the liner in situ within the closure shell.
Although the closures and methods of manufacture disclosed in the noted patents address problems theretofore extant in the art, further improvements remain desirable. For example, although olefin copolymers such as EVA are sufficiently resilient to provide good sealing against the sealing surface of a container when the closure is fastened to the container, these materials do not provide an acceptable barrier against transmission of gases, such as oxygen and carbon dioxide, that can deleteriously affect the product within the container. It has been proposed to employ a barrier material such as ethylene vinyl alcohol (EVOH) as a gas transmission barrier liner. However, materials of this character tend to be expensive and brittle, and are not well suited to function as a seal.
EP 0926078A1 discloses a plastic closure and method of manufacture in which the liner is compression molded in situ on the interior surface of the closure base wall, and consists essentially of a multiplicity of alternating layers of a matrix polymer and a barrier polymer to resist transmission of gas, water vapor and/or flavorants through the liner and closure. An adhesive material is provided in the liner for adhering the matrix polymer layers to the barrier polymer layers, and for adhering the liner to the closure shell, and is either blended with the matrix material or provided as separate layers between the matrix and barrier polymer layers. EVA and EVOH are preferred matrix and barrier polymers respectively. However, it has been found when implementing the teachings of this disclosure that separation and tearing can occur among the matrix and barrier polymer layers when the closure is threaded onto a container finish.
It is therefore a general object of the present invention, in accordance with one aspect of the invention, to provide a liner for a plastic closure that combines the functions of a seal for engagement with the container sealing surface and an improved barrier against gas transmission, flavor absorption (flavor scalping) and/or water vapor permeation. Another object of the present invention, in accordance with a second aspect of the invention, is to provide a barrier liner that is contoured to resist layer separation and tearing as the closure is applied to a container finish.